By the time you read this, JET will be back in operation and in its 31st year. It is remarkable that it is still state-of-the-art, but the combination of excellent design and continuous upgrades has kept JET at the forefront of fusion technology.

The primary focus of the first JET campaigns with the ITER-Like-Wall was to study the interaction of the plasma with beryllium and tungsten plasma facing components. The impact of the materials on plasma behaviour and operating space was also dramatic, with many surprises. Now that the basic JET and ITER operating scenarios have been re-established along with the benefits and challenges of the new materials, the upcoming campaigns will focus on pushing to higher fusion performance by making full use of the upgraded heating power. The programme also includes time to follow through the studies of the most critical physics issues identified in the first campaigns.

A wide variety of experiments have been agreed for the next campaigns. To achieve this, a practical planning assumption has been made that JET will be available about 85 percent of the time, ensuring that there is some flexibility in usage of the machine. This is an important part of planning, as maintenance activities have to be accommodated in the operation of a large and complex plant like this.

Plasma scenarios and impurities

The largest amount of experimental time is dedicated to the development of the JET high performance ‘baseline’ and ‘hybrid’ scenarios – different plasma configurations that promise to be useful as fusion technology develops. In hybrid scenarios the physicists attempt to alter the magnetic field’s ‘twist’ in the plasma in such a way that a lower current is required to achieve high confinement. This has several benefits, including a reduction of the forces that are produced in the event of a plasma disruption. There are still different opinions about how to best implement hybrid scenarios, and much experimental time will be devoted to a better understanding of these plasmas.

The role of impurities in the plasma is also considered highly important and a large proportion of the time will be devoted to studying them. Some experiments will focus on deliberate injection of impurities – nitrogen or neon – to study how heat can be radiated from the plasma edge effectively by them. Other sessions will study the unwanted impurities that are inevitably present in the plasma with a view to minimising their effect on performance. Impurity control has always been an important topic in tokamaks, and it will probably remain so for years to come. With the new metal wall it is found that carbon is no longer the principal impurity; however traces of tungsten can have a strong effect on the core plasma and so control measures are a priority area.

Fuel retention and removal continues to be a hot topic. The new wall components retain much less fuel than the old carbon tiles. Not only are the scientists interested in quantifying this further, but they are interested in finding the best ways to remove trapped fuel. Essentially there are two methods of doing this, namely raising the temperature and bombarding the surface with energetic particles. For ITER this information will prove important for the period where it starts to use tritium.

Deliberate melt-experiments

Perhaps the most surprising experiments are those where tungsten plasma facing components will be deliberately melted. Some tiles have been prepared in such a way that the plasma can be positioned to cause a specific type of damage which is of interest to ITER and only accessible in a large machine such as JET. These tiles have sharp edges that are raised above the normal tile surface, which is the opposite of normal practice where efforts are made to shadow the leading edges behind another tile. A region in the divertor where the plasma does not usually touch has been selected so that the other experiments in the campaign should not be affected. For the melting experiments the strike point of the plasma can be directed onto them. This experiment is not just in the interests of science but a response to a specific request from ITER. It will contribute to the final design of its plasma facing components. It could potentially save ITER a few hundred million Euros if it can be built with a tungsten divertor without having to build one with carbon components for early operation. It seems likely that the JET control room will be well attended for these exciting sessions.

Nick Balshaw, CCFE